This invention pertains to hydrostatic transmission controls for obtaining precise displacement of the variable displacement pump and motor of the transmission and, additionally, for providing accurate, uniform operation as well as steering control of a dual path hydrostatic transmission. The controls are usable in many applications, including track-type vehicles for obtaining both propulsion and steering thereof.
The assignee of this application has a number of prior art patents relating to hydrostatic transmissions. In considering the appropriate prior art, it should be noted that the disclosure herein relates to a hydrostatic transmission control and, more particularly, a dual path hydrostatic transmission control wherein a master circuit including a pressure-reducing valve for setting the control pressure and a directional control valve for establishing the direction of operation of two hydrostatic transmissions as well as a horsepower control valve for preventing stalling of the prime mover connect into two branch circuits with a branch circuit for each of the hydrostatic transmissions and each branch circuit having a steering control valve. The steering control valves provide for modification of the control pressure delivered to the displacement control means of a particular hydrostatic transmission and with a staging action in each transmission whereby the pump of the transmission reaches substantially full displacement prior to reduction in the initial maximum displacement of the motor.
Moon U.S. Pat. No. 3,187,509 discloses a displacement control valve in FIG. 2 which functions as a directional valve and which is in circuit with a manually-controllable torque valve 130. The latter valve functions as a manually-adjustable pressure-reducing valve. This patent does not disclose a dual path control with a master circuit having speed and direction establishing components operable by a single handle and a pair of branch circuits each having a manually-operable steering control valve nor a number of specific features in the displacement control for a transmission providing for precise displacement control.
The Hann et al U.S. Pat. No. 3,230,699 discloses a hydrostatic transmission wherein the reduction in motor displacement is staged to follow the increase in displacement of the pump by having a spring associated with the control valve for the motor stronger than a similar spring in the control valve for the pump. This patent also shows a rotatable shaft with cams for modifying the action of valves.
The Hann U.S. Pat. No. 3,247,669 discloses the use of springs of different strength in the displacement controls for the motor and pump of the hydrostatic transmission in order to stage the operation of the motor after the operation of the pump.
The Hann U.S. Pat. No. 3,411,297 discloses the use of a staging valve 21 for causing change in the displacement of the motor after the pump is adjusted to maximum displacement. This control does not include a shuttle valve for shift in response to a higher control pressure in one of two lines for assuring application of the higher control pressure at all times to one end of the control valve for the motor displacement control. Additionally, this patent shows a feedback linkage associated with the pump.
The Moon U.S. Pat. No. 3,247,919 and Ross U.S. Pat. No. 3,349,860 relate to dual hydrostatic transmissions. In the Moon patent, one handle provides for speed control of both transmissions and a second handle provides for steering. The handles operate control cams for mechanically-modifying the operation of the transmissions. The control cams which set the speed and direction of operation have their actions modified when steering control is desired. In the Ross patent, steering is obtained by use of a separate hydrostatic transmission which operates to provide an input into a differential.
Additionally, the assignee of this application has a previous dual-path hydrostatic transmission control wherein each hydrostatic transmission is manually controlled by a separate control circuit and with each control having a pressure-reducing valve to set a control pressure and a directional control valve for determining the direction of operation of the associated hydrostatic transmission. A horsepower or antistall control valve is connected into the control circuits for both of the hydrostatic transmissions, whereby overloading of the engine will result in a uniform reduction in control pressure in the controls for both of the hydrostatic transmissions. In this prior art control, there are two control handles, with each handle independently setting a speed for a transmission, whereby there is no positive direct relation between the speeds of the two transmissions.
An additional prior art patent is Lauck, U.S. Pat. Re. No. 27,488 wherein two distinct controls, such as described in the preceding paragraph, are operated by a single master control handle. In this patent, the control for each hydrostatic transmission has its own speed control and direction-establishing means as well as a horsepower control valve.